Acrylic rubbers have higher heat resistance and oil resistance than conventional general-purpose rubbers and are in use for various kinds of industrial seal materials. However, these rubbers are not fully satisfactory in compression set and heat resistance temperatures not lower than 170.degree. C. and are unusable for seal materials at such temperatures.
On the other hand, fluororubbers are excellent in heat resistance, compression set, oil resistance and resistance to chemicals and are useful in the field of industrial materials and other fields. Nevertheless, it appears unlikely that these rubbers will be used in remarkably increased quantities in spite of their outstanding properties since economy is not negligible for prevalent use in these fields. Additionally, fluororubbers have the drawback of deteriorating markedly in the presence of additives contained in engine oils.
Investigations were conducted to develop materials having the characteristics of both acrylic rubbers and fluororubbers by blending a common acrylic rubber with a common fluororubber and vulcanizing the blend for molding with use of vulcanizers for one or both of the rubbers (JP-A-40558/1977, JP-A-146752/1978, JP-A- 101847/1979, JP-A-154446/1979, JP-A-156052/1979, JP-A- 23128/1980, JP-A-63740/1983, etc.), whereas the method failed to fully vulcanize one or both of the rubbers, giving products which were unsatisfactory in physical properties such as mechanical strength and compression set, heat resistance, and other properties. Although studies were also made on blends of acrylic rubbers and vinylidene fluoride resins (JP-A-39336/1988, JP-A- 20341/1988, JP-A-19486/1988, JP-A-8447/1988, JP-A- 236841/1987, JP-A-152133/1989 and JP-A-152016/1989), the products still remained to be improved in compression set because vinylidene fluoride resins can not be vulcanized and are crystalline.
In order to overcome these drawbacks, it was proposed to blend a fluororubber with an acrylic rubber intentionally partially crosslinked with a polyfunctional monomer having at least two, preferably three, functional groups, and vulcanize and mold the blend with a vulcanizer for the fluororubber (JP-A- 41379/1994, JP-A-287154/1993 and JP-A-287156/1993), but the disclosed techniques are effective only for compositions comprising a large amount of fluororubber and are not useful practically for compositions containing a large amount of the acrylic rubber i.e., a large amount of partially crosslinked component because of impaired processability such as lower flowability.
On the other hand, it is proposed to prepare a copolymer from an acrylic rubber and dihydrodicyclopentadienyloxyethyl acrylate or dihydrodicyclopentenyl acrylate which are greatly different in reactivity and co-crosslink the acrylic rubber and a fluororubber with a peroxide using the dihydrodicyclopentenyl group remaining in the polymer (JP-A-64143/1990). This method requires the use of two kinds of auxiliary crosslinking agents since difficulty is encountered in effecting crosslinking with an auxiliary crosslinking agent which is useful for both the acrylic rubber and the fluororubber. It is difficult in this case to fully crosslink both the rubbers, which therefore fail to provide a material of satisfactory compression set although the material is acceptable in processability.
A rubber composition is further proposed which comprises a rubber blend of a fluororubber having a reactive bromine group and/or an iodine group in the molecule and an acrylic rubber containing allyl acrylate copolymerized therein, and an organic peroxide and a trialkenyl trimellitic acid ester admixed with the blend (JP-A-245046/1990). The composition contains as much as 4.8 wt. % of allyl acrylate copolymer and is not processable substantially.
Accordingly, none of the compositions containing a large amount of acrylic rubber and heretofore available are satisfactory in all the properties of compression set, heat resistance and processability.
An object of the present invention is to provide a rubber composition having high processability for giving molded products which are excellent in compression set, physical properties such as mechanical strength and heat resistance, and molded products prepared from the composition.